Páginas

5.28.2012

Sunlight Has Changed the Path of Asteroid 1999 RQ36

Scientists with the OSIRIS-REx mission have measure the orbit of the
asteroid 1999 RQ36 with such accuracy that they were able to determine
the drift it experiences after absorbing sunlight and re-emitting the
energy as heat.

The target asteroid is about a third of a mile in diameter (575 meters). Credit: Illustration: NASA/GSFC/UA)

Scientists with the University of Arizona-led asteroid sample return
mission OSIRIS-REx have measured the orbit of their destination
asteroid, 1999 RQ36,
with such accuracy they were able to directly determine the drift
resulting from a subtle but important force called the Yarkovsky effect
-- the slight push created when the asteroid absorbs sunlight and
re-emits that energy as heat.

The new orbit for the half-kilometer (one-third mile) diameter 1999 RQ36
is the most precise asteroid orbit ever obtained, OSIRIS-REx team
member Steven Chesley of the NASA Jet Propulsion Laboratory said. He
presented the findings May 19 at the Asteroids, Comets and Meteors 2012
meeting in Niigata, Japan.

Remarkable observations that Michael Nolan at Arecibo Observatory
in Puerto Rico made in September, along with Arecibo and Goldstone
radar observations made in 1999 and 2005, when 1999 RQ36 passed much
closer to Earth, show that the asteroid has deviated from its
gravity-ruled orbit by roughly 100 miles, or 160 kilometers, in the last
12 years, a deviation caused by the Yarkovsky effect.

The Yarkovsky
effect is named for the 19th-century Russian engineer who first
proposed the idea that a small rocky space object would, over long
periods of time, be noticeably nudged in its orbit by the slight push
created when it absorbs sunlight and then re-emits that energy as heat.

The effect is difficult to measure because it's so infinitesimally small, Chesley said.

The OSIRIS-REx spacecraft will orbit asteroid 1999 RQ36 for a year,
mapping its surface and making detailed measurements. Credit: Illustration: NASA/GSFC/UA)

"The Yarkovsky force on 1999 RQ36 at its peak, when the asteroid is
nearest the Sun, is only about a half-ounce -- about the weight of three
grapes on Earth. Meanwhile, the mass of the asteroid is estimated to be
about 68 million tons. You need extremely precise measurements over a
fairly long time span to see something so slight acting on something so
huge."

Nolan, who obtained his doctorate at the UA, succeeded in a heroic
effort to get a 16-ton power supply for the transmitter from
Pennsylvania to Puerto Rico in six days in time for the observations,
which he made on three separate nights last September. Nolan and his
team measured the distance between the Arecibo Observatory and 1999 RQ36
to an accuracy of 300 meters, or about one-fifth of a mile, when the
asteroid was 30 million kilometers, or 20 million miles, from Earth.

"That's like measuring the distance between New York City and Los
Angeles to an accuracy of 2 inches, and fine enough that we have to take
the size of the asteroid and of Arecibo Observatory into account when
making the measurements," Nolan said.

Chesley and his colleagues used the new Arecibo
measurements to calculate a series of 1999 RQ36 approaches closer to
Earth than 7.5 million kilometers (4.6 million miles) from the years
1654 to 2135. There turned out to be 11 such encounters.

In 2135, the 500-meter (1,640-foot) diameter asteroid will swing by
Earth at around 350,000 kilometers (220,000 miles), its closest approach
over the 481-year time span. That's closer than the Moon,
which orbits about 390,000 kilometers (240,000 miles) from Earth. At
such close distances, the asteroid's subsequent trajectory becomes
impossible to accurately predict so close approaches can only be studied
statistically, Chesley said.

"The new results don't really change what is qualitatively known about the probability of future impacts,"
Chesley said. "The odds of this potentially hazardous asteroid
colliding with Earth late in the 22nd century are still calculated to be
about one in a few thousand."

But the new results do sharpen the picture of how potentially hazardous
1999 RQ36 could be farther into the future. Scientists now have
identified many low-probability potential impacts in the 2170s through
the 2190s while ruling out others, Chesley said.

"OSIRIS-REx
science team members Steve Chesley and Mike Nolan have achieved a
spectacular result with this investigation," said Dante Lauretta, the
mission's principal investigator and professor of planetary science at
the UA. "This study is an important step in better understanding the
Yarkovsky effect -- a subtle force that contributes to the orbital
evolution of new near-Earth objects."

Lauretta added that "this information is critical for assessing the
likelihood of an impact from our target asteroid and provides important
constraints on its mass and density, allowing us to substantially
improve our mission design."

The final piece to the puzzle was provided by the University of
Tennessee's Josh Emery, who used NASA's Spitzer Space Telescope in 2007
to study the space rock's thermal characteristics. Emery's measurements
of the infrared emissions from 1999 RQ36 allowed him to derive the
object's temperatures.

From there he was able to determine the degree to which the asteroid is
covered by an insulating blanket of fine material, which is a key factor
for the Yarkovsky effect.

"1999 RQ36 has about the same density as water, and so it's very light
for its size," said Chesley. "This means that it's more than likely a
very porous jumble of rocks and dust."

Asteroid
1999 RQ36 is of particular interest to NASA as it is the target of the
agency's OSIRIS-REx (Origins, Spectral Interpretation, Resource
Identification, Security, Regolith Explorer) mission. Scheduled for
launch in 2016, ORIRIS-Rex will visit 1999 RQ36, collect samples from
the asteroid and return them to Earth.

NASA detects, tracks and characterizes asteroids and comets passing
close to Earth using both ground and space-based telescopes. The
Near-Earth Object Observations Program, commonly called Spaceguard,
discovers these objects, characterizes a subset of them, and establishes
their orbits to determine if any could be potentially hazardous to our
planet.

Finding the bulk density of a solitary space object by combining radar
tracking and infrared observations might once have seemed almost science
fiction, Chesley said.

What OSIRIS-REx scientists are beginning to learn about Yarkovsky drift
strengthens the idea that "the Yarkovsky effect can be used to probe the
physical properties of asteroids that we can't visit with spacecraft,"
he said.